Not that I've tried anything in practice yet, but I think overlapping UV islands will be problematic for all of the methods mentioned so far. Probably best to convert the target UV set to UDIMs first and then, if the tool used does not support UDIMs directly, convert the texture one tile at a time with the necessary adjustments done based on the UDIM version of the UVs. I hope that makes at least a little bit of sense.

At any rate, since Antonia 1.3 no longer supports the old UV mapping, I've been considering converting some of the textures available for Antonia-A to the standard UV sets. Not sure yet which method I'll try (Blender, an automated variant of FVerbaas' Poser-only method, or an external tool), but if I get anything working, I'll report back here.

-- I'm not mad at you, just Westphalian.